1. Field of the Invention
The present invention relates to a sheet stacking apparatus which includes a plurality of stack trays.
2. Description of the Related Art
In recent years, an image forming apparatus such as a printer and a copying machine has increased in an image forming speed due to advancement in an image forming technique, so that the apparatus can discharge a large amount of sheets in a high speed. Accordingly, a sheet stacking apparatus (hereinafter referred to as a stacker apparatus) which is connected to the image forming apparatus and stacks sheets discharged therefrom is demanded to have large capacity. To meet the demand, a large-capacity stacker apparatus is discussed in which several thousands of sheets discharged from an image forming apparatus can be vertically stacked on a stack tray.
U.S. Patent Application No. 2008/0054558 discusses a stacker apparatus which includes two stack trays arranged side by side, in which A4-size small sheets are stacked on both a left and right stack trays and A3-size large sheets are stacked on across the left and right stack trays (i.e., the two trays). The above stacker apparatus enables a more significant increase in the maximum stack capacity of the small-size sheet with a stack space maintained as it is than a conventional stacker apparatus in which sheets are stacked on a single stack tray without regard to a sheet size and enables an effective use of the stack space.
In the conventional stacker apparatus, the amount of sheets stacked on the stack tray is restricted by a height of stacked sheets. For this reason, if a sheet high in density such as coated paper (which is thin in thickness but heavy in weight) is stacked on a single stack tray to the height by which the amount of sheets to be stacked is restricted, drive torque of a motor for elevating the stack tray may be insufficient. This resultantly causes a problem that a stack operation cannot be continued.
The stacker apparatus discussed in U.S. Patent Application No. 2008/0054558 can ensure a certain amount of a stack capacity by distributing sheets on two stack trays with the number of sheets to be stacked on each stack tray being restricted. Since some users want a configuration of stacking sheets in one column with consideration for workability in the post process, distribution of the sheets on the two stack trays can decrease usability.
Further, the output of a driving motor may be increased and a ratio of a gear for transmitting drive may be changed to avoid a shortage of the drive torque of the motor for elevating the stack tray. This, however, may bring about the disadvantage such as an increase in the cost of the driving motor, and a decrease in usability due to reduction in the elevation speed of the stack tray because of the change of a gear ratio.